Summary. Background: Treatment with Bevacizumab has been associated with arterial thromboembolism in colorectal cancer patients. However, the mechanism of this remains poorly understood, and preclinical testing in mice failed to predict thrombosis. Objective: We investigated whether thrombosis might be the result of platelet activation mediated via the FccRIIa (IgG) receptor -which is not present on mouse platelets -and aimed to identify the functional roles of heparin and platelet surface localization in Bev-induced FccRIIa activation. Methods and results: We found that Bev immune complexes (IC) activate platelets via FccRIIa, and therefore attempted to reproduce this finding in vivo using FccRIIa (hFcR) transgenic mice. Bev IC were shown to be thrombotic in hFcR mice in the presence of heparin. This activity required the heparin-binding domain of BevÕs target, vascular endothelial growth factor (VEGF). Heparin promoted Bev IC deposition on to platelets in a mechanism similar to that observed with antibodies from patients with heparin-induced thrombocytopenia. When sub-active amounts of ADP or thrombin were used to prime platelets (simulating hypercoagulability in patients), Bev IC-induced dense granule release was significantly potentiated, and much lower (sub-therapeutic) heparin concentrations were sufficient for Bev IC-induced platelet aggregation. Conclusions: The prevailing rationale for thrombosis in Bev therapy is that VEGF blockade leads to vascular inflammation and clotting. However, we conclude that Bev can induce platelet aggregation, degranulation and thrombosis through complex formation with VEGF and activation of the platelet FccRIIa receptor, and that this provides a better explanation for the thrombotic events observed in vivo.
Anti-CD40L immunotherapy in systemic lupus erythematosus patients was associated with thromboembolism of unknown cause. We previously showed that monoclonal anti-CD40L immune complexes (ICs) activated platelets in vitro via the IgG receptor (FcγRIIa). In this study, we examined the prothrombotic effects of anti-CD40L ICs in vivo. Because mouse platelets lack FcγRIIa, we used FCGR2A transgenic mice. FCGR2A mice were injected i.v. with preformed ICs consisting of either anti-human CD40L mAb (M90) plus human CD40L, or a chimerized anti-mouse CD40L mAb (hMR1) plus mouse CD40L. ICs containing an aglycosylated form of hMR1, which does not bind FcγRIIa, were also injected. M90 IC caused shock and thrombocytopenia in FCGR2A but not in wild-type mice. Animals injected with hMR1 IC also experienced these effects, whereas those injected with aglycosylated-hMR1 IC did not, demonstrating that anti-CD40L IC-induced platelet activation in vivo is FcγRIIa-dependent. Sequential injections of individual IC components caused similar effects, suggesting that ICs were able to assemble in circulation. Analysis of IC-injected mice revealed pulmonary thrombi consisting of platelet aggregates and fibrin. Mice pretreated with a thrombin inhibitor became moderately thrombocytopenic in response to anti-CD40L ICs and had pulmonary platelet-thrombi devoid of fibrin. In conclusion, we have shown for the first time that anti-CD40L IC-induced thrombosis can be replicated in mice transgenic for FcγRIIa. This molecular mechanism may be important for understanding thrombosis associated with CD40L immunotherapy. The FCGR2A mouse model may also be useful for assessing the hemostatic safety of other therapeutic Abs.
Summary. Background: Tumor-derived tissue factor (TF) activates coagulation in vitro and in vivo in an orthotopic model of human pancreatic cancer. Here, we further characterized tumor-derived TF in this model. Methods: Conditioned medium (CM) of L3.6pl human pancreatic tumor cells and plasma from nude mice bearing L3.6pl tumors were ultracentrifuged, and the pellets were filtered through membranes with different pore sizes. The size distribution of particles was analyzed in CM or plasma fractions with nanoparticle tracking and dynamic light scattering. Human TF antigen and activity were measured in pellets and supernatants with ELISA and clotting or thrombin generation assays, respectively. Human alternatively spliced TF (asTF) was measured with ELISA. Human TF and thrombin-antithrombin complex (TAT) concentrations were assessed in plasma of mice injected with filtered fractions of CM. Results: Particles in both CM and plasma were < 0.4 lm. TF antigen and activity in the CM were mainly associated with microparticles (MP). Approximately 50% of antigen and 20% of activity were associated with particles of < 0.1 lm. Injection of < 0.1-lm particles into mice caused a 30% drop in platelet counts and an increase in TAT levels. In contrast,~90% of TF antigen in tumor-bearing mice plasmas was nonsedimentable, whereas TF activity was exclusively associated with MP. Particles of < 0.1 lm and the supernatants of both CM and plasma gained TF activity after addition of exogenous phospholipids. Although asTF was found in MP-free CM supernatants, it was also present in CM and plasma pellets. Conclusions: Tumor-derived particles of < 0.1 lm and non-sedimentable TF are or can become procoagulant in the presence of phospholipids, and may contribute to the procoagulant potential of circulating TF.
Summary Introduction Platelet activation via the Fcγ receptor IIa (FcγRIIa) is implicated in the pathogenesis of immune complex (IC)-mediated thrombocytopenia and thrombosis (ITT). We previously showed that ICs composed of antigen and antibodies targeting CD40 ligand (CD40L) or β2 Glycoprotein I (β2GPI) induce ITT in mice transgenic for human FcγRIIa (hFcR) but not wild-type controls (which lack FcγRIIa). Here we evaluated the contribution of the guanine nucleotide exchange factor, CalDAG-GEFI, and P2Y12, key regulators of Rap1 signaling in platelets, to ITT induced by these clinically relevant ICs. Methods Pre-formed anti-CD40L or anti-β2GPI ICs were injected into hFcR/Caldaggef1+/+ or hFcR/Caldaggef1-/- mice, with or without clopidogrel pre-treatment. Animals were observed for symptoms of shock for 30 minutes, during which time core body temperature was monitored. Platelet counts were obtained before and 30 minutes after IC injection. Lungs were assessed for thrombosis by histology or near-infrared imaging. Results Both CD40L and β2GPI ICs rapidly induced severe thrombocytopenia, shock and a reduction in body temperature in hFcR/Caldaggef1+/+ mice. hFcR/Caldaggef1-/- mice were protected from CD40L and β2GPI IC-induced thrombocytopenia and shock, whereas P2Y12 inhibition had only a modest effect on IC-induced ITT. Consistent with these findings, IC-induced integrin activation in vitro and the accumulation of activated platelets in the lungs of IC-challenged mice was strongly dependent on CalDAG-GEFI. Conclusions Our studies demonstrate that CalDAG-GEFI plays a critical role in platelet activation, thrombocytopenia and thrombosis induced by clinically relevant ICs in mice. Thus, CalDAG-GEFI may be a promising target for the intervention of IC-associated, FcγRIIa-mediated thrombotic conditions.
The multifunctional cytokine, TWEAK (TNF-like weak inducer of apoptosis), is a member of the TNFα superfamily. TWEAK is found in a broad range of cell types and has been linked to cell growth and survival, angiogenesis and other inflammatory processes. These functions and their importance in inflammatory diseases have made TWEAK an attractive pharmaceutical target, particularly for immunotherapy with monoclonal antibodies (mAbs). Immunotherapy targeting another TNFα family member, CD154, was associated with thrombosis in clinical trials. Subsequent studies identified platelets, which contain CD154, as a possible contributing factor to thrombosis in these trials. Since clinical trials with anti-TWEAK mAbs have already begun, we considered it important to determine whether platelets contain TWEAK. Using a variety of immunologic methods we found that, upon activation, human platelets expose TWEAK antigen and release it in soluble form (sTWEAK). By flow cytometry we determined that human platelets activated by TRAP (Thrombin Receptor Agonist Peptide) and other agonists expose TWEAK antigen (22% median positivity) and release TWEAK positive microparticles. The presence of TWEAK on platelets was confirmed by confocal microscopy. By ELISA, we found that sTWEAK is released by activated platelets. Finally, western blot analysis revealed TWEAK protein (34 kDa) in washed platelet lysates. The finding that human platelets contain TWEAK raises important questions about its possible functions in normal physiology, as well as in inflammatory diseases and their treatment.
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